The document FR-A-2 493 826 discloses a process for calcining gypsum in which a bed of gypsum is heated by direct introduction of hot gas and is maintained at a temperature appropriate for the production of calcium sulphate hemihydrate, water being sprayed into the hot gases before the latter come into contact with the gypsum.
The document EP-A-230 793 discloses a process and a device for calcining calcium sulphate dihydrate. In this document, combustion gases are introduced directly via a first tube extending downwards into a bed of material to be calcined. The very hot combustion gases are cooled before coming into contact with the material of the bed to be calcined. For this, cooler recycling gases, gypsum or both are introduced into a second tube partially surrounding the first tube, thus absorbing a portion of the heat of the hot gases by conduction through the wall of the first tube. The gypsum comes into contact with the material of the bed before coming into contact with the hot gases.
There are disadvantages to this device. The gypsum introduced between the two tubes has a tendency to build up on the tubes and to become clogged, thus blocking the calcining device. This problem is particularly significant when the material to be calcined is synthetic gypsum, for example gypsum from flue gas desulphurization.
The calcining device disclosed in the document EP-A-0 284 464 comprises a first tube for introducing hot air into the material of a bed to be calcined. This first tube is surrounded by a second tube. Fresh material to be calcined passes through the intermediate space between the first tube and the second tube. Baffles are placed between the first tube and the second tube in the path of the fresh material.
There are disadvantages to this device. This device requires pre-drying of the gypsum before it is introduced into the calcining device, in order to prevent clogging. In addition, this device requires crushed natural gypsum which has to be milled at the outlet of the furnace; this device is therefore not appropriate for gypsum from flue gas desulphurization, and synthetic gypsums more generally.
These devices also have disadvantages in common. The dehydration of the gypsum is insufficient and nonhomogeneous. In addition, the thermal efficiency of these devices is reduced. Furthermore, the excess pressure inside these devices requires safety measures and additional equipment, such as booster pumps, with high installation and maintenance costs. Generally, the times for calcining the gypsum in these plants are high, resulting in low throughputs in comparison with their size and a low reactivity of the hemihydrate obtained.